TTT,
Having done both SAR and NS, there is no comparison between the environments. The only commonality is the sea. The bottom line is that when NS, if you loose an engine, you will fly away; at times in SAR, if you loose an engine you will crash. That is why SAR pilots wear helmets. And certainly in the machines I have flown, I have never knocked a switch with my helmet, even in the mountains where the turbulence is far greater than anything in the NS.
Helmets are bloody uncomfortable in hot weather and in my opinion offer poorer noise attenuation than a standard headset.
For my money, STASS before helmets.